The present invention relates to a liquid crystal display element including a spacer which is sandwiched between a pair of substrates made of plastic and the like, and a manufacturing method thereof.
Conventionally, as a pair of substrates for composing a liquid crystal display element by holding a liquid crystal, substrates made of plastic and the like have been used because of superior impact resistance. However, a substrate made of plastic and the like has a problem of an inferior gas barrier, i.e., gas is easily transmitted through the substrate, thereby arising such a problem that the gas transmitted through the substrate dissolves in a liquid crystal layer so as to allow bubbles to be generated more easily.
More specific explanation is provided as follows. When composing a liquid crystal display element, a polarizer, etc. are attached to a substrate. Since the substrate and the polarizer generally have different elongation ratio, due to environmental change (change in a temperature, humidity, etc.) is deformed the substrate to a bimetallic form. A cell gap between the pair of substrates is changed due to the deformation of the substrate, thereby changing pressure between the pair of substrates sandwiching the liquid crystal. In addition, the change in the pressure causes the gas that dissolved in the liquid crystal layer to be vaporized, and the bubbles are generated.
On the other hand, a technique disclosed in Japanese Unexamined Patent Publication No. 281941/1994 (Tokukaihei 6-281941 published on Oct. 7, 1994), etc., is such that, by increasing the number of spacer sandwiched by the pair of substrates, deformation of the substrate particularly under a high temperature and high humidity is reduced, thereby attempting to suppress generation of the bubbles in the case of using a substrate of an inferior gas barrier.
Further, the foregoing publication also discloses a technique of adopting a spacer of an adhesive type, suppressing movement of the spacer by adhering the spacer to the substrate in a manufacturing process, and preventing the deformation of the substrate.
Note that, such a liquid crystal display element is commonly formed by injecting a liquid crystal between a pair of substrates, surroundings of which are sealed with a sealing agent by a vacuum impregnation method, except a liquid crystal injection slit (liquid crystal impregnation slit), and sealing the liquid crystal injection slit with end seal (sealing agent) made of UV setting resin, etc. Here, for example, a technique disclosed in Japanese Unexamined Patent Publication No. 142506/1993 (Tokukaihei 5-142506 published on Jun. 11, 1993), etc., is such that, after injecting the liquid crystal, a panel which is made up of a pair of substrates is pressed so as to squeeze out the liquid crystal, then, the liquid crystal injection slit is sealed, thereby uniforming the cell gap between the pair of substrates.
However, in the case of using the spacer of the adhesive type disclosed in the publication No. 6-281941, as the spacer of the adhesive type is adopted a spacer, a surface of which is coated with a thermoplastic adhesive. In that case, the unit cost of the spacer becomes high, thereby arising a problem of increase in the cost of a liquid crystal display element.
Further, in that case, adhesion of the spacer to the substrate is carried out simultaneously with the heat treatment for setting a seal material in the manufacturing process, but when using a UV setting resin as the seal material, a heat treatment process should separately be provided, thereby arising problems of complicated processes and increase in cost.
Further, in that case, since the thermoplastic adhesive exists inside the liquid crystal, the thermoplastic adhesive dissolves in the liquid crystal when performing the heat treatment, thereby possibly resulting in exacerbation of the performance of the liquid crystal.
Further, in the technique disclosed in the publication No. 6-281941, an effect is produced with respect to the generation of bubbles due to the deformation of the substrate under a high temperature and high humidity. However, when the number of the spacers is excessively large, a relative quantity of the liquid crystal inside the cell is reduced. In such case where the relative quantity of the liquid crystal is small, and particularly, in case where the liquid crystal is kept under a condition of a low temperature and the liquid crystal itself shrinks largely, there arises such a problem that a mechanical impact from outside allows bubbles to be readily generated in a vacuum region (a space where the liquid crystal is eliminated and substantially no gases are included).
Furthermore, when the number of the spacers is excessively large, a problem of reduction in contrast of the liquid crystal display element arises.
On the other hand, when the number of the spacers is excessively small, the relative quantity of the liquid crystal inside the cell is increased, and the degree of generation of bubbles due to the mechanical impact (characteristic of resistance to a mechanical impact bubbles) is increased. However, the small number of the spacers results in a reduction in a support between the substrates. Consequently, there arises problems such as readily deformation of a substrate, deterioration of resistance to pressure, and nonuniformity in cell gap.
Particularly, when using a substrate which is made of a material such as plastic having smaller stiffness than that of glass and the like, it is necessary to attain stiffness of the liquid crystal display element by increasing a spacer density to a certain extent.
Meanwhile, in the case of an STN (Super-Twisted Nematic) type liquid crystal cell which requires highly accurate adjustment of cell gap, it is crucial to maintain the cell gap uniformly. Specifically, in the STN type liquid crystal cell, a product xcex94nd (retardation) of a cell gap d and birefringence xcex94n of a liquid crystal injected inside the cell determines background color gradations, and therefore a partial variation in the cell gap d results in color shading.
In order to uniform the cell gap d, the application of pressure to a panel prior to sealing of the liquid crystal injection slit as disclosed in the publication No. 5-142506 is effective, but when the pressure here is high, the liquid crystal is discharged excessively. Therefore, when the panel after sealing is left to stand in an environment of a low temperature, a liquid crystal inside the panel shrinks so as to form bubbles in the vacuum region inside the panel.
Further, in the case of a panel using a plastic substrate in particular, when the spacer density (spacer scattering density) is low, as shown in FIG. 7, a substrate is bent in accordance with presence or absence of the spacer. FIG. 7 is a cross sectional view showing a state of a liquid crystal display element 10 including spacers 4 whose density is low. In the liquid crystal display element 10, a display side substrate 1a and a rear side substrate 1b are bent inward at a portion where an interval between the spacers 4 is wide, and thereby a cell of this portion becomes small. This is recognized as color shading in the case of the STN type liquid crystal cell. Furthermore, the spacers 4 can uniformly support the display side substrate 1a and the rear side substrate 1b when the spacers 4 are completely spaced equidistantly, but it is difficult to attain such complete equidistant spacing by a common spacer scattering technique. Consequently, since the support by the spacers 4 varies depending on a location, the cell gap fails to be uniform, thereby occasionally emphasizing the color shading.
On the other hand, when the spacer density is high, bending of the display side substrate 1a and the rear side substrate 1b is suppressed, and moreover, the addition of pressure allows the spacers 4 to support the substrates uniformly, and thus it is relatively easy to uniform the cell gap. However, since a quantity of a liquid crystal occupying a space inside the cell decreases, the bubbles in the vacuum region can be readily formed when left to stand in the environment of a low temperature.
The present invention is attained so as to solve the foregoing problems, and it is an object thereof to provide a liquid crystal display element having a desirable display quality of a display image, avoiding complication of manufacturing processes to the utmost while securing stiffness of a liquid crystal panel, suppressing fluctuations in a cell gap due to deformation of a substrate when applying pressure so as to suppress occurrence of color shading, and further, suppressing reduction in contrast and generation of bubbles in a vacuum region due to mechanical impact from outside under a low temperature, and to provide a manufacturing method thereof.
In order to attain the foregoing object, the liquid crystal display element according to the present invention includes: a pair of substrates made of plastic; a liquid crystal sandwiched between the pair of substrates; and a plurality of spacers for maintaining a gap d between the pair of substrates, which are sandwiched between the pair of substrates; wherein, when x is an average value of thickness of the plurality of spacers in a state without a load in such a direction that the plurality of spacers are sandwiched between the pair of substrates, d less than xxe2x89xa61.1d is satisfied.
With the foregoing arrangement, the plurality of spacers are provided so as to maintain the distance d between the pair of substrates for holding the liquid crystal therebetween, and the average thickness x of the plurality of spacers (provided that the thickness in such a state that is not held between the pair of substrates without a load) (hereinafter referred to as xe2x80x9cthickness of spacers xxe2x80x9d) is larger than the distance d. Namely, when the liquid crystal display element is arranged, the pair of substrates and the plurality of spacers are compressed in a direction of the thickness.
Therefore, stress by compression occurs in a contact portion of a display side substrate and a rear side substrate making up the pair of substrates, and the plurality of spacers. This stress produces a force to act in a direction of the thickness of the plurality of spacers, and friction between the pair of substrates and the plurality of spacers, which acts in the direction of the surface of the pair of substrates, is increased by the force.
Consequently, with this arrangement, the spacers are more firmly fixed on the pair of substrates than an arrangement in which the cell gap d and the particle diameter of the spacers are the same, thereby suppressing the movement of the spacers.
Accordingly, with this arrangement, without using adhesive type spacers to which an adhesive is applied as in a conventional arrangement, the spacers can firmly be fixed by changing adjustment of the size of the spacers and the distance d between the substrates in manufacturing processes. Therefore, problems such as complication of the manufacturing processes as in the case of using the adhesive type spacers can be prevented.
Thus, by firmly fixing the spacers, in such a case that a display surface of the liquid crystal display element is pressed, variations in the distance d between the substrates due to the movement of the spacers can be prevented, thereby suppressing occurrence of color shading in an displayed image due to the variations in the distance d between the substrates.
Meanwhile, when the thickness of the spacers x exceeds 1.1 times the distance d between the substrates, the amount of deformation of substrates and spacers becomes excessively large. Consequently, problems such as difficulty in uniformly adjusting the distance between the substrates to be a predetermined value, and damages in transparent electrodes formed on the substrates may arise.
On the other hand, with the foregoing arrangement, the thickness x of the spacers is set to be not more than 1.1 times the distance d between the substrates, thereby preventing excessive deformation of the substrates and spacers, and the foregoing problems can be prevented.
Thus, with the foregoing arrangement, in the liquid crystal display element, by setting the thickness x of the spacers to be larger than the distance d between the substrates and not more than 1.1 times the distance d between the substrates, problems such as the complication of the manufacturing processes and damages in the transparent electrodes can be prevented, while firmly fixing the spacers, thereby suppressing generation of color shading, thus improving display quality of the liquid crystal display element.
Further, in the liquid crystal display element of the present invention, it is preferable that a numerical density of the spacers is in a range of not less than 240 pieces/mm2 and not more than 300 pieces/mm
In the liquid crystal display element, when the numerical density of the spacers becomes large, a relative quantity of the liquid crystal is decreased. In a state that the relative quantity of the liquid crystal is small, as discussed, when a liquid crystal display element under a low temperature is given a mechanical impact from outside, bubbles in the vacuum region are likely to be generated.
With the foregoing arrangement, the numerical density of the spacers is set to be not more than 300 pieces/mm2, thereby suppressing generation of bubbles in the vacuum region due to decrease in the relative quantity of the liquid crystal.
Further, when the numerical density of the spacers is not more than 300 pieces/mm2, deterioration in contrast of the liquid crystal display element caused by scattering of light due to the spacers can also be prevented.
With the foregoing arrangement, further, the numerical density of the spacers is set to be not less than 240 pieces/mm2. Consequently, stiffness of the liquid crystal display element is improved, and the pair of substrates are less likely to be deformed when pressure is applied to the display surface, thereby suppressing variations in the distance d between the substrates, and further suppressing generation of color shading.
Thus, with the foregoing arrangement, generation of bubbles in the vacuum region due to decrease in the relative quantity of the liquid crystal, reduction in contrast, and generation of color shading can be suppressed, thus improving display quality of the liquid crystal display element.
In the liquid crystal display element of the present invention, further, it is preferable that an elastic modulus of the spacers is larger than that of the pair of substrates.
With the foregoing arrangement, since the elastic modulus of the spacers is larger than that of the substrates, at a portion where the spacers and the pair of substrates come in contact, the amount of deformation in the substrates increases, thereby forming concave portions on the surfaces of the substrates which were originally flat. Namely, the spacers support the substrates while slightly sinking into the substrates.
In case where the surfaces of the substrates are flat, the spacers are held mainly by friction between the spacers and the substrates. However, when the concave portions are formed on the surfaces of the substrates, the spacers are more firmly held not only by friction but by sinking into the concave portions.
Consequently, with the foregoing arrangement, an effect of suppressing the movement of the spacers is further improved, thereby further effectively suppressing generation of color shading, thus improving display quality of the liquid crystal display element.
In order to attain the foregoing object, a manufacturing method of a liquid crystal display element which includes a pair of substrates made of plastic, a plurality of spacers for maintaining a gap between the pair of substrates and a liquid crystal between the pair of substrates, including the steps of:
(a) disposing the plurality of spacers between the pair of substrates, and forming a liquid crystal injection slit for injecting the liquid crystal between the pair of substrates while bonding the pair of substrates with a seal material which is provided around a region where the plurality of spacers are disposed;
(b) injecting the liquid crystal between the pair of substrates via the liquid crystal injection slit; and
(c) sealing the liquid crystal injection slit while adjusting pressure which is applied to the pair of substrates so as to adjust the gap between the pair of substrates in which the liquid crystal was injected,
wherein:
when x is an average value of thickness of the plurality of spacers in a state without a load in such a direction that the plurality of spacers are sandwiched between the pair of substrates,
when adjusting the pressure, the gap d between the pair of substrates at completion of the liquid crystal display element is adjusted so as to satisfy d less than xxe2x89xa61.1d.
With the foregoing arrangement, by adjusting pressure by applying pressure to the substrates to which the liquid crystal was injected and adjusting the pressure, the distance d and the thickness x of the spacers at the time of completion of the liquid crystal display element is set to satisfy d less than xxe2x89xa61.1d. Consequently, since the substrates and the spacers are in a state of being compressed in directions of the thickness, thereby, as discussed, much firmly fixing the spacers. Accordingly, problems such as complication of the manufacturing processes can be prevented while improving display quality of the liquid crystal display element by a relatively easy method.
The manufacturing method of the liquid crystal display element of the present invention, further, it is preferable to dispose the spacers between the pair of substrates so that its numerical density is in a range of not less than 240 pieces/mm2 and not more than 300 pieces/mm2.
With the foregoing arrangement, as discussed, generation of bubbles in the vacuum region due to decrease in the relative quantity of the liquid crystal can be suppressed, while increasing stiffness and improving display quality and reliability of the liquid crystal display element by a relatively easy method.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.